A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as carbon monoxide and sulfur oxides, in waste gas streams from the processing and combustion of fuels. Specifically, refinery operations associated with the fluidized catalytic cracking of petroleum and petroleum derivatives typically result in the production of waste gas streams which contain relatively high levels of both carbon monoxide and sulfur oxides. The present invention is directed to a method for the reduction of these emissions without the need for complex and expensive waste gas treatment facilities.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. Illustrative of fluidized catalytic cracking processes is the method wherein suitably preheated high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated riser reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons. Suitable hydrocarbon feeds generally boil within the range from about 400.degree. to about 1,200.degree. F. and are usually cracked at temperatures ranging from 850.degree. to 1,050.degree. F.
In a catalytic process of this type, some non-volatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons which generally contain 4 to 10 weight percent hydrogen. As coke accumulates on the catalyst surfaces, the catalyst activity and selectivity diminish.
Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from catalyst surfaces with an oxygen containing gas such as air. The oxidation of these carbonaceous deposits of coke may be regarded, in a simplified manner, as the oxidation of carbon according to the following equations: EQU (1) C + O.sub.2 .fwdarw. CO.sub.2 EQU (2) 2c + o.sub.2 .fwdarw. 2co EQU (3) 2co + o.sub.2 .fwdarw. 2co.sub.2
reactions (1) and (2) both occur under typical catalyst regeneration conditions wherein the catalyst temperature may range from about 1,050.degree. to about 1,300.degree. F. The combustion of carbon monoxide to carbon dioxide according to reaction (3) proceeds only at temperatures above about 1,100.degree. F. Consequently, the incomplete combustion of carbon monoxide during catalyst regeneration can result in significant concentrations of carbon monoxide in the regeneration zone effluent gas. The discharge of this carbon monoxide into the atmosphere is undesirable, not only from an environmental point of view, but also because it represents a wasted source of heat energy. The combustion of carbon monoxide yields approximately 4,350 B.T.U. per pound.
When high-sulfur feedstocks, such as petroleum hydrocarbons containing sulfur compounds, are utilized in a catalytic cracking unit, the coke deposited on the catalyst contains sulfur. During regeneration of the coked deactivated catalyst, the coke is burned from the catalyst surfaces and results in conversion of the sulfur to sulfur dioxide together with small amounts of sulfur trioxide. These sulfur oxides are thus discharged in the regeneration zone effluent gas stream. The processing of a high-sulfur feedstock often results in emissions of sulfur oxides in the regeneration zone effluent which are in the range of about 1,200 parts per million.
Although catalyst regeneration operations typically generate substantial emissions of carbon monoxide and sulfur oxides, other refinery operations also produce significant quantities of these undesirable emissions. Waste gas streams from boilers and other process furnaces in the refinery typically contain undesirable emissions of carbon monoxide and sulfur oxides. Refinery tail gases may also contain emissions of carbon monoxide and sulfur oxides.
A variety of methods have been described for removing sulfur oxides from waste gas streams. These include aqueous washing or scrubbing, chemical absorption, neutralization, and chemical reaction or conversion. These methods are not entirely satisfactory, however. Unfortunately, they all require complex and expensive auxiliary equipment which results in increased operating and capital costs.
Another approach, which is limited to the removal of sulfur oxides from the regenerator effluent gas stream in a cyclic fluidized catalytic cracking process, is set forth in U.S Pat. No. 3,835,031. This approach contemplates the use of a molecular sieve type cracking catalyst which is impregnated with one or more Group IIA metal oxides. The metal oxides react with sulfur oxides in the regeneration zone to form non-volatile inorganic sulfur compounds. These non-volatile inorganic sulfur compounds are then converted to hydrogen sulfide in the reactor and stripper zones of the cyclic process. The hydrogen sulfide is removed by conventional means. Similarly, U.S. patent applications Ser. No. 642,541; 642,542; 642,544; and 642,545 also contemplate the elimination of sulfur oxides from catalyst regenerator effluent gas by the use of a molecular sieve type catalyst in combination with a metal-containing component which reacts with sulfur oxides.
Still another approach, which is also limited to the removal of sulfur oxides from the regenerator effluent gas stream in a cyclic fluidized catalytic cracking process, is set forth in U.S. Pat. No. 3,699,037. This approach involves the addition of at least a stoichiometric amount of a calcium or magnesium compound to the cracking cycle in relation to the amount of sulfur deposited on the catalyst. This added material is intended to react with the resulting sulfur oxides and then, being in a finely divided condition, exit from the cracking cycle as particulate material in the regenerator effluent gas stream. Continued addition of such material obviously increases operating costs, and merely substitutes one form of pollutant for another.
The removal of carbon monoxide from waste gas streams has been accomplished through the use of carbon monoxide boilers and associated means for recovery of the energy released by conversion of the carbon monoxide to carbon dioxide. Such methods, however, require complex auxiliary equipment which serves to increase operating and capital costs.
The removal of carbon monoxide from regenerator effluent gas streams in cyclic fluidized catalytic cracking processes has been the subject of substantial inventive effort. U.S. Pat. No. 3,909,392 describes the essentially complete oxidation of carbon monoxide to carbon dioxide within the regenerator vessel under conditions which permit substantially complete recovery of the evolved heat by direct transfer to catalyst particles. This patent also discloses the use of oxidation promoters within the regeneration zone, which serve to accelerate the combustion of carbon monoxide. Similarly, Belgian Pat. No. 826,266 is directed to a method which involves a catalytic cracking catalyst in physical association with a carbon monoxide-oxidation promoter which is a metal having an atomic number of at least 20 and may be selected from Groups IB, IIB, and III to VIII of the Periodic Table. Further, U.S. Pat. No. 3,808,121 discloses the regeneration of a cracking catalyst in the presence of a carbon monoxide-oxidation promoter which is retained in the regeneration zone.
Belgian Pat. No. 7,412,423 discloses that a cracking catalyst containing less than 100 parts per million, calculated as metal and based on total catalyst, of at least one metal component selected from the group consisting of metals selected from Periods 5 and 6 of Group VIII of the Periodic Table, rhenium, and compounds thereof, is effective in reducing the carbon monoxide content of effluent gases derived from the regeneration of catalytic cracking catalysts.
U.S. Pat. applications Ser. No. 642,542 and 642,545 describe the reduction of carbon monoxide levels in effluent gases from the regeneration of catalytic cracking catalyst through the use of a cracking catalyst which is combined with a carbon monoxide-oxidation promoter and a metal-containing component which reacts with sulfur oxides. In addition, U.S. patent application Ser. No. 642,533 contemplates the use of a carbon monoxide-oxidation promoter and an oxidation stabilizer to reduce carbon monoxide levels in effluent gases derived from catalyst regeneration.
Although relatively simple and inexpensive methods are now available for the control of emissions of carbon monoxide and sulfur oxides derived from catalyst regeneration in cyclic fluidized catalytic cracking processes, such methods are not available for the control of such emissions in tail gases or waste gas streams from boilers and other process furnaces associated with the refinery. The method used for reducing these emissions should not be dependent upon the nature of the fuel undergoing combustion and should permit full utilization of the heat energy available through combustion of the carbon monoxide emissions. It is also necessary that the method selected not substitute one form of undesirable waste for another, such as reducing emissions of sulfur oxides coupled with an increase in particulate emission. It is further desirable that the method of control not require significant operating or capital costs. Finally, it is critical that the method selected for reducing such emissions be effective without lowering the activity and selectivity of the cracking catalyst.